The present invention relates to an image forming apparatuses, such as electronic photographs and facsimile machines, and more particularly to a tandem-type image forming apparatus for driving a plurality of image bearing members arranged on a line by one endless image-bearing member drive belt.
Recently, there is known, as an image forming apparatus, a tandem-type image forming apparatus having a plurality of image bearing members for different colors arranged on a line. In such a tandem-type image forming apparatus, even where there is a speed variation between the image bearing members, it is important to correctly align the colors based on each image through the image bearing members. For this reason, by adjusting the phases of the image bearing members and suppressing misalignment of colors, it is possible to align the colors for each image with further correctness. In such a case, when the image bearing members are rotatively driven and adjusted in their phases by gear trains, there encounters an increased number of gears thus requiring a greater space.
For this reason, there is a proposal of an image forming apparatus which has a plurality of image bearing members for different colors to be driven by one endless image bearing member drive belt and formed compact while suppressing misalignment of colors by adjusting the phases of the image bearing members (see JP-B-2806617, for example).
In the image forming apparatus disclosed in JP-B-2806617, a timing belt as one endless image bearing member drive belt is wound over a driving pulley on an output shaft of a drive motor, an image bearing member pulley for yellow (Y), an image bearing member pulley for magenta (M) and an image bearing member pulley for cyan (C). The drive force of the drive motor is transmitted to the image bearing member pulleys by the timing belt. Furthermore, by adjusting the mesh points between the image bearing member pulleys and the timing belt, the phase of positional deviation amount of an image is matched on the image bearing members thus preventing against the occurrence of misalignment of colors. In this case, the timing belt is also wound over a plurality of idler pulleys. Although the function of these pulleys are not definite, there is no disclosure as to having a function of tension pulleys. It can be considered that they are for merely setting up a belt path in order to provide a predetermined mesh at between the timing belt and the image bearing member pulleys.
Meanwhile, there is also a disclosure of an image forming apparatus formed compact which has a plurality of image bearing members to be driven by one endless image bearing member drive belt and formed compact while suppressing concentration irregularities and color misalignment by keeping uniform the behavior of the image bearing members (see JP-A-2002-304033, for example).
In the image forming apparatus disclosed in JP-A-2002-304033, a timing belt as one endless image bearing member drive belt is wound over a driving pulley, black (K) image bearing member pulley, magenta (M) image bearing member pulley and cyan (C) image bearing member pulley to which the power of the drive motor is conveyed. By providing a tension to the timing belt through an idler pulley as one belt tension tightening unit and the tension pulley of another of belt tension tightening unit, the construction is made simple and moreover the plurality of image bearing members are kept even in rotational behavior, thus obtaining a quality image reduced in concentration irregularities and color misalignment.
Furthermore, there is a proposal of an image forming apparatus which has a plurality of image bearing members wound over by timing belts as a plurality of endless image bearing member drive belts, to provide a tension to the timing belts by idler pulleys and convey the drive force of the drive motor by the timing belts to the image bearing member pulleys, wherein a movement time required for the image bearing member to move up to a post-exposure transfer position is set integer times the time required for the driving pulley provided on the drive motor end to rotate once, thereby obtaining a quality color image free of color misalignment occurrence (see JP-A-5-72837, for example).
The image forming apparatus disclosed in JP-A-5-72837 is set with an exposure-to-transfer time on the image bearing member that is integer times a rotation period of the driving pulley, thereby enabling drive conveyance by image bearing members free from the rotation irregularities of the drive motor appearing as image misalignment. In this case, the plurality of idler pulleys for adjusting a tension of the timing belt has a rotation period set equal to the rotation period of the driving pulley, thus preventing against positional deviations of a transfer image due to rotation irregularities of the idler pulleys.
In the meanwhile, in the image forming apparatus disclosed in JP-B-2806617 that the plurality of image bearing members to be driven by the one timing belt merely set with a belt path as noted above through the idler pulleys, the construction is comparatively simple but the tension of the timing belt is not to be adjusted constant. Thus, rotational behavior is not even over the plurality of image bearing members. Therefore, it can be considered in the image forming apparatus disclosed in JP-B-2806617 that, by providing a tension to the timing belt through a tension pulley and keeping the belt tension constant as in the image forming apparatus disclosed in JP-A-2002-304033, rotational behavior is to be kept uniform over the plurality of image bearing members.
However, where using the idler pulley in this manner, the idler pulley involves an eccentricity due to geometrical error, mounting error, etc. In case there is an eccentricity of the idler pulley in this manner, the idler pulley at its outer peripheral surface is to reciprocally move relative to the timing belt during rotation of the idler pulley. Consequently, there is caused a change in the belt path of the timing belt contacting with the outer peripheral surface of the idler pulley, causing speed variations on the timing belt. Because the speed of the image bearing member varies due to the speed variation on the timing belt, transfer images deviate in position thus causing color misalignment. In this case, in the image forming apparatus using the idler pulleys of the image forming apparatus disclosed in JP-B-2806617 and the tension pulley of the image forming apparatus disclosed in JP-A-2002-304033, the driving pulley is connected to the drive motor. Consequently, when the idler pulley at its outer peripheral surface moves reciprocally as in the foregoing, there is a not so much movement in a belt portion positioned closer to the driving pulley than the idler pulley while there is a comparatively great movement in a belt portion positioned closer to the tension pulley than the idler pulley. For this reason, the occurrence of speed variation is greater on the timing belt in a portion positioned closer to the driving pulley than the idler pulley.
Therefore, it can be considered to prevent against the positional deviation of images due to rotational irregularities of the idler pulleys by applying the provision of an exposure-to-transfer time on the image bearing member that is integer times a rotation period of the driving pulley and the setting the rotation period of the idler pulley equal to the rotation period of the driving pulley in the image forming apparatus disclosed in JP-A-5-72837, to an image forming apparatus using such idler pulley and tension pulley as in the foregoing. However, the idler pulley of the image forming apparatus disclosed in JP-A-5-72837 is a tension pulley instead of an idler pulley for merely setting up a belt path as in the foregoing, to which the image forming apparatus disclosed in JP-A-5-72837 cannot be applied. Moreover, because the rotation period of the idler pulley of the image forming apparatus disclosed in JP-A-5-72837 is given equal to the rotation period of the driving pulley, rotation period is restricted on the idler pulley.
By the way, there is a proposal of an image forming apparatus that yellow (Y), magenta (M), cyan (C) and black (K) of image bearing members are mutually positioned in preset reference positions and supported for rotation on an image bearing member support member, to rotatively drive the image bearing members by a belt, wherein a positioning pin is provided on a side plate of the image bearing member support member to thereby acquire color misalignment information with reference to a position of the positioning pin so that, when the image bearing members are deviated in relative position, correction for color misalignment is effected under registration control on the basis of the color misalignment information (see JP-A-2003-280313, for example).
According to the color-misalignment correction disclosed in JP-A-2003-280313, correction is possible for color misalignment resulting from the accuracy in support position of image bearing member shafts, the working accuracy of these shafts, the accuracy in diameter of the image bearing members and the like.
Meanwhile, there is a proposal of an image forming apparatus that register marks for detecting image positions are formed, color by color, at a constant interval over a transfer belt by an electronic photographic process separately from the images to be formed on the transfer member, to read the register marks by a CCD sensor so that correction is made for positional deviation of from a preset reference position between images depending upon an image signal read out (see JP-B-2603254, for example).
The image forming apparatus disclosed in the JP-B-2603254 is adapted to easily correct for positional deviation in a direction of transfer material conveyance (top margin), positional deviation in a scanning direction (left margin), inclination deviation in an oblique direction and image-to-image deviation resulting from magnification error, by the register marks, thus making it possible to form a quality image.
As disclosed in JP-A-2002-304033 and JP-B-2603254, correct-controlling the positional deviation from a preset reference position between a plurality of images to thereby correct for color misalignment due to the positional deviation is referred, in the invention, to as correction for color misalignment based on registration control.
In the meanwhile, because of the presence of a contact between the image bearing members and a transfer member of a transfer device and a speed difference between the peripheral speed of the image bearing member and the peripheral speed of the transfer member, a resistance force is caused to act between the image bearing member and the transfer member. Because of an elongation caused in the image bearing member drive belt by the resistance force, positional deviations occur in the pre-adjusted phase condition of the image bearing members to thereby cause color misalignment in each image.
However, in the image forming apparatuses disclosed in JP-B-2806617 and JP-A-2002-304033, prevention is made against the color misalignment in each image caused by the image bearing members through keeping an adjusted phase of the image bearing member pulleys and the timing belt. Nevertheless, no consideration is made at all as to the foregoing color misalignment in the image due to an elongation of the image bearing member drive belt. Particularly, in the image forming apparatus having an intermediate transfer belt to which toner images on the image bearing members are transferred, a resistance force is caused to act also upon the intermediate transfer belt due to the friction between the image bearing member and the intermediate transfer belt. Due to this, because color misalignment is caused in the mage due to an elongation of the intermediate transfer belt, there is a problem of a partial increase of color misalignment due to the respective elongations of the both belts.
Further, in the image forming apparatuses disclosed in JP-B-2806617 and JP-A-2002-304033, the driving pulley is provided greatly spaced from image bearing member pulleys arranged tandem on a line, in a line perpendicular to an arrangement line of the image bearing member pulleys. Consequently, because the belt length of the image bearing member drive belt is increased, the image bearing member drive belt is increased in elongation amount, therefore, the color misalignment is further increased. Moreover, despite the use of a belt provides compactness as compared to gear train driving to the image bearing members, the arrangement structure of the driving pulley requires a large arrangement space thus raising a problem of impeding the precious compactness.
Furthermore, although the image forming apparatus disclosed in JP-A-2003-280313 can correct for misalignment of colors resulting from the accuracy in support position of image bearing member shafts, the working accuracy of these shafts, the accuracy in diameter of the image bearing members and the like, it does not yet consider color misalignment in each image due to an elongation of the image bearing member drive belt and an elongation of the intermediate transfer belt.
Furthermore, the image forming apparatus disclosed in JP-B-2603254, although corrects for positional deviations between a plurality of images by resister marks, does not yet consider the elongation of the image bearing member drive belt and image-based color misalignment due to elongation of the intermediate transfer belt.